1. Field of the Invention
The invention relates to the catalytic copolymerization of ethylene with other copolymerizable monomers in a fluid bed reactor to produce low (0.900 to 0.925) and medium (0.926 to 0.940) density ethylene copolymers.
2. Description of the Prior Art
The commercialization of low and medium density ethylene polymers is much more significant in the United States, and in the rest of the world, than is the commercialization of high density (&gt;0.940) ethylene polymers. The low density polymers are usually made commercially by homopolymerizing ethylene with free radical catalysts under very high pressures (.gtorsim.15,000 psi) in tubular and stirred reactors, in the absence of solvents. The medium density polymers can also be made commercially by the high pressure process, or by blending high pressure polyethylene with high density polyethylene made in a low pressure process with transition metal based catalysts.
The preparation of ethylene polymers in the absence of solvents under low pressures (.about.40-350 psi) in a fluid bed reactor, using various supported chromium containing catalysts, is disclosed in U.S. Pat. Nos. 3,023,203; 3,687,920; 3,704,287; 3,709,853, Belgium Pat. No. 773,050 (which corresponds to U.S. Patent Application S.N. 156,187 filed June 24, 1971, and abandoned in favor of continuation-in-part application S.N. 426,559 filed Dec. 20, 1973) and Netherlands Patent Application 72-10881. These publications also disclose that the ethylene polymers produced may be ethylene homopolymers or copolymers of ethylene and one or more other alpha olefins.
The disclosures in these patent publications are primarily concerned with the preparation of high density ethylene polymers. Furthermore, because of the technical difficulties involved, it has not been possible, prior to the present invention, to provide a commercially useful process for the production of low and medium density ethylene polymers in a low pressure fluid bed process. In order to provide such a useful process, the catalyst employed must be one which can, simultaneously, copolymerize ethylene with other alpha olefins so as to provide the desired density range in the copolymer product; provide a polymer product having a particle size which is conductive to being readily fluidized in a fluid bed reactor; provide such a high degree of productivity that the catalyst residues in the polymer product are so small as to allow them to remain therein, and thus avoid the need for catalyst removal steps; provide a polymer product which can be readily molded in a variety of molding applications, i.e., provide a polymer product having a relatively wide melt index range; provide a polymer product which has a relatively small low molecular weight fraction content so as to allow the product to meet Federal Food and Drug Administration standards for extractables (&lt;5.5 weight percent at 50.degree. C. in n-hexane) for food contact applications; and be used in solid form to provide such copolymer products under the operating conditions which can be readily achieved in a commercial sized fluid bed reactor.
Thus, attempts to use various prior art catalysts, for the purposes of attempting to make low to medium density ethylene polymers in a commercially useful fluid bed process have not been successful to date, since such catalysts do not provide the desired combination of features noted above. For example, certain Ziegler catalysts provide products which have a very narrow melt index range (0.0 to .about. &lt;0.2), and are readily subject to catalyst poisoning and have a relatively low productivity as evidenced by a catalyst residue content of greater than about ten parts per million of transition metal. The Ziegler polymers thus usually have to undergo a catalyst residue removal operation.
The supported bis(cyclopentadienyl)chromium [II] catalysts disclosed in U.S. Pat. No. 3,687,920; U.S. Pat. No. 3,709,853 and Belgian Pat. No. 773,050 do not readily allow for the copolymerization of enough of the suitable comonomers with ethylene to provide copolymers having densities below about 0.945.
Although the family of supported silyl chromate catalysts disclosed in U.S. Pat. No. 3,324,095, U.S. Pat. No. 3,324,101 and U.S. Pat. No. 3,704,287 will provide ethylene copolymers of relatively low density, some of the catalysts in the family will provide polymer products which have a high content of small particle sizes which cannot be readily fluidized, and/or which have a narrow melt index range (0.0 to .about. &lt; 0.2), and/or which have a relatively high n-hexane extractables content.
The supported chromium oxide catalysts disclosed in U.S. Pat. Nos. 2,825,721 and 3,023,203 can be used to provide low and medium density ethylene copolymers provided that relatively high ratios of comonomer to ethylene are employed in the monomer feed stream. However, the copolymers produced have a relatively narrow melt index range (0.0 to .about. &lt;0.2). The supported titanated chromium oxide catalysts disclosed in U.S. Pat. No. 3,622,521 provide copolymers which have a relatively high level of extractables (in n-hexane at 50.degree. C.) therein.
Netherlands Patent Application 72-10881 discloses the use of a supported fluorided and titanated chromium oxide catalyst for ethylene polymerization purposes, but does not disclose a practical method for making low to medium density ethylene copolymers in a commercially useful fluid bed process.
Thus, based on the technology known prior to the present invention, it was not possible to make low to medium density ethylene polymers having a relatively high melt index of &gt;0.0 to about 2.0, a relatively low n-hexane soluble fraction and a relatively low residual catalyst content at relatively low temperatures and pressures in the absence of solvent in a commercially useful fluid bed process.